We characterize the thermal conductivity of graphite, monolayer graphene, graphane, fluorographane, and bilayer graphene, solving exactly the Boltzmann transport equation for phonons, with ...phonon–phonon collision rates obtained from density functional perturbation theory. For graphite, the results are found to be in excellent agreement with experiments; notably, the thermal conductivity is 1 order of magnitude larger than what found by solving the Boltzmann equation in the single mode approximation, commonly used to describe heat transport. For graphene, we point out that a meaningful value of intrinsic thermal conductivity at room temperature can be obtained only for sample sizes of the order of 1 mm, something not considered previously. This unusual requirement is because collective phonon excitations, and not single phonons, are the main heat carriers in these materials; these excitations are characterized by mean free paths of the order of hundreds of micrometers. As a result, even Fourier’s law becomes questionable in typical sample sizes, because its statistical nature makes it applicable only in the thermodynamic limit to systems larger than a few mean free paths. Finally, we discuss the effects of isotopic disorder, strain, and chemical functionalization on thermal performance. Only chemical functionalization is found to play an important role, decreasing the conductivity by a factor of 2 in hydrogenated graphene, and by 1 order of magnitude in fluorogenated graphene.
This study focuses on examining the influence of bast fibers on the flammability and thermal properties of the polylactide matrix (PLA). For this purpose, Urtica dioica and Vitis vinifera fibers were ...subjected to two types of modifications: mercerization in NaOH solution (M1 route) and encapsulation in an organic PLA solution (M2 route). In a further step, PLA composites containing 5, 10, and 15 wt% of unmodified and chemically treated fibers were obtained. The results of the tests show that only biocomposites containing mercerized fibers had a nearly 20% reduced flammability compared to that of PLA. Moreover, the biofiller obtained in this way belongs to the group of flame retardants that generate char residue during combustion, which was also confirmed by TGA tests. The M2 modification route allowed to achieve higher mass viscosity than the addition of unmodified and M1-modified fibers. The reason is that fibers additionally encapsulated in a polymer layer impede the mobility of the chain segments. The inferior homogenization of the M2-modified fibers in the PLA matrix translated into a longer combustion time and only a 15% reduction in flammability.
A controlled alignment of graphite nanoplatelets in a composite matrix will allow developing materials with tailored thermal properties. Achieving a high degree of alignment in a reproducible way, ...however, remains challenging. Here we demonstrate the alignment of graphite nanoplatelets in copper composites produced via high-energy ball milling and spark plasma sintering. The orientation of the nanoplatelets in the copper matrix is verified by polarized Raman scattering and electron microscopy showing an increasing order with increasing platelet size. The thermal conductivity k along the alignment direction is up to five times higher than perpendicular to it. The composite with the highest degree of alignment has a thermal diffusivity (100 mm2s–1) comparable to copper (105 mm2s–1) but is 20% lighter. By modeling the thermal properties of the composites within the effective medium approximation we show that (i) the Kapitza resistance is not a limiting factor for improving the thermal conductivity of a copper-graphite system and (ii) copper-graphite-nanoplatelet composites may be expected to achieve a higher thermal conductivity than copper upon further refinement.
The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a microbridge device with built-in resistance thermometers. Based on the measured thermal resistance ...values of 11–12 atomic layer h-BN samples with suspended lengths ranging between 3 and 7.5 μm, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 W m–1 K–1, approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 W m–1 K–1 at 300 K. Thermal conductivities for both the 5-layer and the 11-layer samples are suppressed at low temperatures, suggesting increasing scattering of low frequency phonons in thin h-BN samples by polymer residue.
Nanofluids are considered to offer important advantages over conventional heat transfer fluids. Over a decade ago, researchers focused on measuring and modeling the effective thermal conductivity and ...viscosity of nanofluids. Recently important theoretical and experimental research works on convective heat transfer appeared in the open literatures on the enhancement of heat transfer using suspensions of nanometer-sized solid particle materials, metallic or nonmetallic in base heat transfer fluids. The purpose of this review article is to summarize the important published articles on the enhancement of the forced convection heat transfer with nanofluids.
•Elastic properties of titanium borides are calculated by first principles calculation.•Thermodynamical stability of titanium borides is analyzed.•Heat capacity and thermal expansion coefficient for ...titanium borides are calculated and compared.•Grüneisen parameters of titanium borides are calculated.
The anisotropic elastic and thermal expansions of the titanium borides (TiB2, Ti3B4, TiB_Pnma and TiB_Fm3¯m) are calculated from first-principles using density functional theory. All borides show different anisotropic elastic properties; the bulk, shear and Young’s moduli are consistent with those determined experimentally. The temperature dependence of thermal expansions is mainly caused by the restoration of thermal energy due to phonon excitations at low temperature. When the temperature is higher than 500K, the volumetric coefficient is increased linearly by increasing temperature. Meanwhile, the heat capacities of titanium borides are obtained based on the knowledge of thermal expansion coefficient and the elasticity, the calculations are in good agreement with the experiments.
The Boudouard reaction, which is the reaction of carbon and carbon dioxide to produce carbon monoxide, represents a simple and straightforward method for the remediation of carbon dioxide in the ...environment through reduction: CO2(g) + C(s) ⇌ 2CO. However, due to the large positive enthalpy, typically reported to be 172 kJ/mol under standard conditions at 298 K, the equilibrium does not favor CO production until temperatures >700 °C, when the entropic term, −TΔS, begins to dominate and the free energy becomes negative. We have found that, under microwave irradiation to selectively heat the carbon, dramatically different thermodynamics for the reaction are observed. During kinetic studies of the reaction under conditions of flowing CO2, the apparent activation energy dropped from 118.4 kJ/mol under conventional convective heating to 38.5 kJ/mol under microwave irradiation. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction dropped from 183.3 kJ/mol at ∼1100 K to 33.4 kJ/mol at the same temperature under microwave irradiation. This changes the position of the equilibrium so that the temperature at which CO becomes the major product drops from 643 °C in the conventional thermal reaction to 213 °C in the microwave. The observed reduction in the apparent enthalpy of the microwave driven reaction, compared to what is determined for the thermal reaction from standard heats of formation, can be thought of as arising from additional energy being put into the carbon by the microwaves, effectively increasing its apparent standard enthalpy. Mechanistically, it is hypothesized that the enhanced reactivity arises from the interaction of CO2 with the steady-state concentration of electron–hole pairs that are present at the surface of the carbon due to the space-charge mechanism, by which microwaves are known to heat carbon. Such a mechanism is unique to microwave-induced heating and, given the effect it has on the thermodynamics of the Boudouard reaction, suggests that its use may yield energy savings in driving the general class of gas–carbon reactions.
This paper presents a study on the thermal properties of a range of geopolymers in order to assess their suitability for high temperature applications such as thermal barriers, refractories and fire ...resistant structural members. Geopolymers were synthesised from five different fly ashes using sodium silicate and sodium aluminate solutions to achieve a set range of Si:Al compositional ratios. The thermo-physical, mechanical and microstructural properties of the geopolymers are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. The amount and composition of the amorphous component (glass) of each of the fly ashes was determined by combining XRD and XRF results. It was found that the Si:Al ratio in the glass of the fly ashes strongly influenced the thermal performance of the geopolymers. Geopolymers synthesised from fly ashes with a high Si:Al (≥5) in the glass exhibited compressive strength gains and greater dimensional stability upon exposure to 1000°C, whereas geopolymers synthesised from fly ashes with low Si:Al (<2) in the glass exhibited strength losses and reduced dimensional stability upon high temperature exposure.
► Quantification of the crystalline and amorphous phases of five different fly ashes. ► Si:Al ratio in the glass of the fly ash influences the thermal performance. ► Compressive strength of geopolymers can increase by up to 5 fold after firing. ► Sodium aluminate activated geopolymers exhibit thermal stability up to 800°C. ► Sintering during thermal exposure improved inter-particle bonding.
Based on the fact that tumor cells are more thermolabile and less oxygen resistant than normal cells, a novel minimally invasive tumor treatment modality, magnetic hyperthermia (MHT) is proposed. ...However, magnetic materials, which are necessities in MHT, are toxic to human tissue if they are excessively employed. Thus, magnetic materials with higher specific absorption rate (SAR) and intrinsic loss power (ILP) values becomes imperative to minimize their clinical application while mitigating damage to normal human tissues under identical treatment temperature conditions. In this study, the incorporation of soft magnetic MnFe2O4 into hard magnetic CoFe2O4 was introduced, leveraging the exchange coupling between these materials to enhance the magnetic saturation (Ms). Subsequently, we combined these magnetic nanomaterials with graphene oxide (GO) to establish an efficient heat conduction pathway, further augmenting the specific absorption rate (SAR) value of the resulting magnetic composites. Our findings reveal that the Mn0.5Co0.5Fe2O4-1%GO composite developed in this investigation boasts superior SAR values and intrinsic loss power (ILP) of 49.7 W/g and 5.90 nHm2/kg, respectively, when compared to other magnetic materials. The nanomaterials synthesized in this study have the potential to significantly enhance the effectiveness of tumor magnetothermal therapy.
•Improve the SAR by increasing the magnetic saturation with exchange coupling between the soft and hard magnetic phases.•Combine magnetic nanomaterials with graphene oxide to construct heat transfer channels to further improve the SAR.•Investigate Mn1-xCoxFe2O4 MNPs in a wide range of Mn and Co ratios and select the best one with highest SAR value.•Investigate magnetic properties of Mn0.5Co0.5Fe2O4-GO MNPs with different GO contents in alternating magnetic field.
Graphene consists of a monolayer of sp2 bonded carbon atoms and has attracted considerable interest over recent years due to its extreme mechanical, electrical, and thermal properties. Graphene ...nanocomposites have naturally begun to be studied to capitalize upon these properties. A range of complex chemical and physical processing methods have been devised that achieve isolated graphene sheets that attempt to prevent aggregation. Here we demonstrate that the simple casting of a polymer solution containing dispersed graphene oxide, followed by thermal reduction, can produce well-isolated monolayer reduced-graphene oxide. The presence of single layer reduced-graphene oxide is quantitatively demonstrated through transmission electron microscopy and selected area electron diffraction studies and the reduction is verified by thermogravimetric, X-ray photoelectron spectroscopy, infrared spectrum, and electrical conductivity studies. These findings provide a simple, environmentally benign and commercially viable process to produce reduced-graphene oxide reinforced polymers without complex manufacturing, dispersion or reduction processes.
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